Children have been subjected to rape and prostitution by United Nations peacekeepers in Haiti and Liberia, a BBC investigation has found.
Girls have told of regular encounters with soldiers where sex is demanded in return for food or money.
A senior official with the organisation has accepted the claims are credible.
The UN has faced several scandals involving its troops in recent years, including a DR Congo paedophile ring and prostitute trafficking in Kosovo.
The assistant secretary-general for peacekeeping operations acknowledges that sexual abuse is widespread.
"We've had a problem probably since the inception of peacekeeping - problems of this kind of exploitation of vulnerable populations," Jane Holl Lute told the BBC.
"My operating presumption is that this is either a problem or a potential problem in every single one of our missions."
The UN is scheduled to hold a special conference in New York on Monday 4 December, to address the issue.
The BBC inquiry was commissioned as part of Generation Next - a week of programmes focusing on people under 18.
The BBC's Mike Williams with a teenager who claims she was raped by a serviceman
In Haiti, the BBC's Mike Williams spoke to a street girl as young as 11 who had reported sexual abuse by peacekeepers outside the gates of the presidential palace in Port-au-Prince.
A 14-year-old described her abduction and rape inside a UN naval base in the country two years ago.
Despite detailed medical and circumstantial evidence, the allegation was dismissed by the UN for lack of evidence - and the alleged attacker returned to his home country.
In Liberia, meanwhile, a 15-year-old said she had been attacked by a UN officer on 15 November.
In May this year, another BBC investigation discovered systematic abuse in Liberia, involving food being given out to teenage refugees in return for sex.
The UN responded by heightening policing measures, appointing 500 monitors across the country, and introducing mandatory training of all personnel on appropriate conduct.
A local NGO worker said reports of sexual abuse involving peacekeepers were "still rampant, despite pronouncements that they have been curbed".
'Culture of silence'
UN chief Kofi Annan has pledged a policy of "zero tolerance".
The UN's own figures show 316 peacekeeping personnel in all missions have been investigated, resulting in the summary dismissal of 18 civilians, repatriation of 17 members of Formed Police Units and 144 repatriations or rotations home on disciplinary grounds.
However allegations remain that measures to police and curb misconduct are nowhere near as strong as they should be.
Refugees International says there remains a "culture of silence" in some military deployments, and fear of punishment is not enough to ensure compliance with UN rules.
"They may be military men but they are also humanitarian workers," Sarah Martin told the BBC.
"To prey upon the very populations that you are sent to protect is one of the worst forms of violation and betrayal that there is."
Under UN regulations, military personnel cannot be prosecuted in the country where they are serving, and it is up to the courts in their home countries to prosecute crimes committed.
The UN said it had firm knowledge of only two concrete examples of sex offenders being sent to jail, although it believed there could be others it did not know about.